Complement is the first line of defense against infections and plays a role in shaping the repertoire of the immune response. However, uncontrolled and excessive complement activation significantly contributes to diverse pathologies ranging from inflammation, autoimmune tissue injury and injury that follows ischemia and reperfusion events. We will use a mesenteric ischemia/reperfusion (IR) model of tissue injury in an attempt to understand the mechanisms of local intestinal damage associated with complement activation and recruitment and activation of neutrophils. Because immunoglobulin deficient, RAG-1-/- mice and complement receptor 2 deficient, Cr2-/- mice, are protected from IR-induced injury and because some Cr2-/- mice have limited numbers of B1 B cells, we asked whether protection of Cr2-/- mice from IR-induced injury could be reversed by normal antibodies. Transfer of normal IgG and IgM into Cr2-/- mice completely restores IR- induced damage with each isotype transferring unique aspects of the damage. Therefore, we hypothesize that certain autoantibodies, produced by Bland B1-like B cells and constituting part of the natural antibody repertoire, recognize cryptic antigens expressed by stressed or injured tissues, activate complement and induce tissue damage. In this proposal, we will test our hypothesis with experiments grouped into three specific aims: 1. To determine the antigenic specificity of IgG natural antibodies that bind to injured tissue and activate complement. 2. To determine the role of CR2 and its ligand in the production of these antibodies. 3. To determine the specific B population that does not produce the damaging antibodies in Cr2- /- mice. These experiments will elucidate the role of complement and antibodies in tissue injury and the mechanism of its activation. During severe injury or organ transplantation, the blood supply to the intestine is decreased causing a condition known as ischemia. When the blood supply is returned (reperfusion), components of the immune system attack the intestine and cause additional damage. This application proposes experiments designed to reveal the components of the immune system that cause the damage. These studies will provide critical data to improve our understanding of ischemia/reperfusion injury that occurs not only in the intestine but will also be applicable to other human conditions such as during heart attacks and strokes.